A conventional oil pump has an inner rotor formed with n external teeth where n is a natural number, an outer rotor formed with n+1 internal teeth that mesh with the external teeth, and a casing with an suction port which draws in fluid and a discharge port which discharges fluid. The outer rotor is rotated by rotating the inner rotor with the external teeth meshed with the internal teeth, which causes the volumes of a plurality of cells formed between the rotors to change to draw in or discharge the fluid.
The cells are individually separated by the virtue of the fact that external teeth of the inner rotor and internal teeth of the outer rotor contact at forward and rearward positions with respect to the rotating direction respectively, and of the fact that the both side surfaces are sealed by the casing, thereby forming individual fluid conveying chambers. And after the volume attains its minimum in the process of the engagement between the external teeth and the internal teeth, the volume of each cell increases to draw in fluid as it moves along the suction port, and after the volume attains its maximum, the volume decreases to discharge fluid as it moves along the discharge port.
Because of their small size and simple structure, the oil pumps having the above configuration are broadly used as pumps for lubricating oil, or for automatic transmissions, etc. in cars. When incorporated in a car, a crankshaft direct connect actuation is used as an actuating means for the oil pump, in which the inner rotor is directly linked with the engine crankshaft, and is driven by the rotation of the engine.
Incidentally, various types of oil pumps have been disclosed including the type which uses an inner rotor and an outer rotor in which the tooth profile is defined by a cycloid, (for example, see Patent Document 1), the type which uses an inner rotor in which the tooth profile is defined by an envelope for circular arcs that are centered on a trochoid (for example, see Patent Document 2), or the type which uses an inner rotor and an outer rotor in which the tooth profile is defined by two circular arcs in contact with each other, (for example, see Patent Document 3), and also an oil pump which uses an inner rotor and an outer rotor in which the tooth profile of each type described above is modified.
In recent years, the discharge capacity of the oil pump is on an increase due to a trend to make the driven valve system adjustable and due to an addition of the oil jet for piston cooling with increasing engine power. On the other hand, the miniaturization and reduction in the radius of the body of the oil pump are desired to reduce engine friction from the viewpoint of reducing the fuel cost. While it is common to reduce the number of teeth to increase the discharge amount of the oil pump, since the discharge amount per cell increases in an oil pump with a small number of teeth, the pulsation becomes more pronounced and there was the problem of noise due to vibration of pump housing etc.
While it is common to increase the number of teeth as a way to reduce pulsation and to suppress noise, if the number of teeth is increased with teeth having the tooth profile defined by a theoretical cycloid etc., the amount of discharge will decrease. And, in order to secure the required amount of discharge, either the outside radius of the rotor or the thickness needs to be increased, which results in problems such as increased size and weight or friction.                Patent Document 1: Japanese Patent Application Publication No. 2005-076563        Patent Document 2: Japanese Patent Application Publication No. H09-256963        Patent Document 3: Japanese Patent Application Publication No. S61-008484        